(a) Field of the Invention
The present invention relates generally to ultrasonic transducer assemblies and, more particularly, to transducer assemblies of the composite or sandwich type incorporating a deformable pressure element.
(b) Description of the Prior Art
Ultrasonic transmission devices are well known for use in a variety of applications such as, for example, surgical operations and procedures. Ultrasonic transmission devices usually include a transducer that converts electrical energy into vibrational motion at ultrasonic frequencies. The vibrational motion is transmitted to vibrate a distal end of a surgical instrument. Such uses are disclosed in representative U.S. Pat. Nos. 3,636,943 and 5,746,756, both incorporated herein by reference.
High-intensity ultrasonic transducers of the composite or sandwich type typically include front and rear mass members with alternating annular piezoelectric transducers and electrodes stacked therebetween. Most such high-intensity transducers are of the pre-stressed type. They employ a compression bolt that extends axially through the stack to place a static bias of about one-half of the compressive force that the piezoelectric (PZT) transducers can tolerate. Sandwich transducers utilizing a bolted stack transducer tuned to a resonant frequency and designed to a half wavelength of the resonant frequency are described in United Kingdom Patent No. 868,784. When the transducers operate they are designed to always remain in compression, swinging from a minimum compression of nominally zero to a maximum peak of no greater than the maximum compression strength of the material.
As shown in FIG. 1, an acoustic or transmission assembly 80 of an ultrasonic device generally includes a transducer stack or assembly 82 and a transmission component or working member. The transmission component may include a mounting device 84, a transmission rod or waveguide 86, and an end effector or applicator 88.
The transducer assembly 82 of the acoustic assembly 80 converts the electrical signal from a generator (not shown) into mechanical energy that results in longitudinal vibratory motion of the end effector 88 at ultrasonic frequencies. When the acoustic assembly 80 is energized, a vibratory motion standing wave is generated through the acoustic assembly 80. The amplitude of the vibratory motion at any point along the acoustic assembly 80 depends on the location along the acoustic assembly 80 at which the vibratory motion is measured. The transducer assembly 82, which is known as a “Langevin stack”, generally includes a transduction portion 90, a first resonator or aft end bell 92, and a second resonator or fore end bell 94. The transducer assembly 82 is preferably an integral number of one-half system wavelengths (nλ/2) in length.
The distal end of the first resonator 92 is connected to the proximal end of transduction section 90, and the proximal end of the second resonator 94 is connected to the distal end of transduction portion 90. The first and second resonators 92 and 94 have a length determined by a number of variables, including the thickness of the transduction section 90, the density and modulus of elasticity of material used in the resonators 92 and 94, and the fundamental frequency of the transducer assembly 82.
The transduction portion 90 of the transducer assembly 82 may include a piezoelectric section (“PZTs”) of alternating positive electrodes 96 and negative electrodes 98, with piezoelectric elements 70 alternating between the electrodes 96 and 98. The piezoelectric elements 70 may be fabricated from any suitable material, such as, for example, lead zirconate-titanate, lead meta-niobate, lead titanate, or ceramic piezoelectric crystal material. Each of the positive electrodes 96, negative electrodes 98, and piezoelectric elements 70 have a bore extending through the center. The positive and negative electrodes 96 and 98 are electrically coupled to wires 72 and 74, respectfully. The wires 72 and 74 transmit the electrical signal from the generator to electrodes 96 and 98.
The piezoelectric elements 70 are energized in response to the electrical signal supplied from the generator to produce an acoustic standing wave in the acoustic assembly 80. The electrical signal causes disturbances in the piezoelectric elements 70 in the form of repeated small displacements resulting in large compression forces within the material. The repeated small displacements cause the piezoelectric elements 70 to expand and contract in a continuous manner along the axis of the voltage gradient, producing high frequency longitudinal waves of ultrasonic energy. The ultrasonic energy is transmitted through the acoustic assembly 80 to the end effector 88.
The piezoelectric elements 70 are conventionally held in compression between the first and second resonators 92 and 94 by a bolt and washer combination 106. The bolt 106 preferably has a head, a shank, and a threaded distal end. The bolt 106 is inserted from the proximal end of the first resonator 92 through the bores of the first resonator 92, the electrodes 96 and 98, and piezoelectric elements 70. The threaded distal end of the bolt 106 is screwed into a threaded bore in the proximal end of second resonator 94.
Other embodiments of the prior art utilize a stud that is threadedly engaged with both the first and second resonators 92 and 94 to provide compressive forces to the PZT stack. Threaded studs are also known in the prior art for attaching and detaching transmission components to the transducer assembly. See, for example, U.S. Pat. Nos. 5,324,299 and 5,746,756. Such bolts and studs are utilized to maintain acoustic coupling between elements of the sandwich type transducer or any attached acoustic assembly. Coupling is important to maintain tuning of the assembly, allowing the assembly to be driven in resonance.
In previous designs, the compression means may be inadequate and may be unable to provide a uniform pressure across the inside diameter to the outside diameter of each PZT and through the entire PZT stack, the “r” and “z” axes as shown in FIG. 1 and graphically illustrated in FIG. 2. A Finite Element analysis shows that the ratio of the pressure in the R axis is of the order of 4:1.
U.S. Pat. No. 5,798,599 discloses an ultrasonic transducer assembly which includes soft, aluminum foil washers disposed between facing surfaces of adjacent members of the PZT stack. The washers deform under compressive loading to follow the microscopic surface irregularities of the adjacent member surfaces. However, such washers are used primarily to address local stresses and do not address the macroscopic stress gradients present in loaded ultrasonic instruments.
Current designs, such as those disclosed in U.S. Pat. No. 6,491,708 to Madan, et al. attempt to provide a more uniform distribution across individual PZTs and through the PZT stack. One such disclosed embodiment includes providing a bolt head that is substantially equal to the diameters of the individual PZTs. A second disclosed embodiment provides for an aft end bell having a first contact surface and a second contact surface, where the contact area of the second contact surface is less than the surface area of the first contact surface. Rather than applying pressure to the PZT stack at the central bore of the bolt hole, as has been provided in previous devices, the disclosed embodiment transfers the applied pressure to a location offset from the central bore. The Madan patent discloses an improvement in Finite Element Analysis (FIG. 3) over prior designs, such as, for example, the pressure distribution illustrated in FIG. 2, yet may require the use of non-standard components.
FIG. 2 illustrates a prior art plane 10 defined by points 12, 14, 16, and 18 illustrating an example of a Finite Element Analysis for conventional transducer designs. FIG. 2a illustrates that plane 10 is a plane extending radially from the central axis of transducer assembly 82, excepting the bore, and extends longitudinally from the most proximal surface of the transduction portion 90 to the most distal surface of the transduction portion 90. FIG. 3 illustrates a Finite Element Analysis of U.S. Pat. No. 6,491,708 to Madan, et al. showing an improved uniform pressure distribution over a transduction portion plane 20. Transduction portion plane 20 corresponds to plane 10 and illustrates an improved uniform pressure distribution across the proximal surface of the transduction portion as is shown between points 22 and 24.
Non-uniform pressure across the r and z axes may reduce transducer efficiency and may lead to high heat generation. This limitation becomes acutely critical in temperature-limited applications. In temperature-limited applications, the reduced efficiency translates into higher heat generation in the transducer and reduced maximum output. Further, non-uniform pressure limits the magnitude of compression and therefore limits the power capability of the transducer.
There is a need, therefore, for an ultrasonic transducer provided with standard components that exhibits substantially uniform compressive stresses across each PZT and throughout the PZT stack to reduce heat generation and increase power output efficiency.